The ever increasing needs for the storage of large quantities of data in modern computer systems have required the development of new techniques for information storage. Optical techniques, such as holography, permit high density information storage greater than that attainable with conventional magnetic recording. In a holographic memory, the information stored in the hologram is stored uniformly throughout the hologram rather than in discrete areas. The hologram is thus relatively insensitive to blemishes or dirt on the memory medium. A small blemish or dust particle on the memory medium cannot obscure a bit of digital data as it can in a bit-by-bit memory.
As background, in the co-pending application Ser. No. 424,991, filed Dec. 14, 1973, titled "Holographic Memory with Moving Memory Medium" by M. F. Braitberg and T. C. Lee, and assigned to the same assignee as the present invention the basic technique of storing one-dimensional (one-d) holograms using a one-d page composer as the input is described. The teachings of that application are incorporated by reference. In that reference each such one-d hologram formed one track on the moving tape.
Briefly explained, the referenced co-pending application is a holographic optical memory in which many holograms are stored on a moving memory medium. In a holographic memory, each hologram represents a different bit pattern or "page" and a single hologram may represent a page containing as many as 10.sup.5 bits. Information is stored by directing an information beam and a reference beam to a desired location on the memory medium. The information beam, which contains a bit pattern formed by a page composer, interferes with the reference beam to form the hologram. To read out the information, a readout beam selectively illuminates one of the holograms stored, thereby producing a reconstructed image of the bit pattern stored in the hologram. An array of photodetectors is positioned to detect the individual bits of the reconstructed bit pattern.
The holographic system of the reference application uses mutually coherent signal and reference beams to store one-dimensional Fourier transform holograms on a moving memory medium. A page composer creates an elongated bit pattern in the signal beam. This elongated bit pattern contains a plurality of bits arranged in an essentially linear array. Fourier Transform means produces a one-dimensional Fourier Transform of the elongated bit pattern at the memory medium. The mutually coherent signal and reference beams interfere at the memory medium to store a one-dimensional Fourier Transform hologram having interference fringe lines and an elongated dimension. The interference fringe lines run parallel to the direction of motion of the memory medium, and the elongated dimension is essentially orthogonal to the direction of motion. An advantage of the fringe lines running parallel to tape motion is that it allows more exposure time which means minimum peak power requirement from the laser source. In the referenced application only the optics of a single track format is shown.
This invention relates to holography, and in particular to passive optics for a multi-track holographic tape recorder, that is, to optics for utilizing a multiplicity of linear page composers and for storing the data as one-dimensional holograms in a multiple track format on a moving tape. In so doing, the full tape surface becomes available storage area and a large amount of data can be stored on tapes holographically at rates up to 10 gigabits/sec.